There are many situations in integrated circuit (IC) processing where resistors of different values are required for various circuit functions. In particular, the measurement of temperature requires resistors with relatively high temperature coefficient of resistance (TCR) and low noise. While metal films do not have the highest TCR, they do have very low intrinsic noise, and thus can be used advantageously in the measurement of small temperature changes. One particular application is in the detection of infrared radiation using micro-electro-mechanical (MEMS) type structures and bolometric measurements or calorimetric measurement using MEMS type structures.
A problem with the use of such metal film resistors arises during the fabrication process for forming such resistors in the IC. In such fabrication and other compatible processes the thin films of metal—such as titanium, Ti— exhibit a degradation of their TCR characteristics on exposure to the high temperatures that are encountered during these processes. Conventionally formed as thin films (typically 100-1000 nm), the metal films show initial good TCR levels (about 0.25-0.35% C−1) when first deposited but suffers degradation during the subsequent processing to levels that are often not suitable for the intended application. There are several considered reasons as to why this may occur including theories based on micro-structural changes (grain size and growth) which may affect the TCR and also that an incorporation of oxygen and/or nitrogen into the formed metal film may influence the observed TCR. The growth of the micro-crystallites is dependent on:    a) the underlying “bed” material, specifically its smoothness,    b) high temperature processing steps post deposition of the metal layer that can directly impact grain growth,    c) contaminant species such as nitrogen or oxygen, argon or other doping materials.
The contaminant species impact grain growth by usually clustering to the grain boundaries and changing the energy required to allow grain growth. They also have different carrier transport properties thus can give rise to changes in the resistive properties of the material and subsequently the TCR. The film thickness itself is well known to have an affect on the overall resistive properties. FIG. 1 shows, in simplified form, a typical cross section through a multilayer semiconductor structure 100. The structure includes a base substrate material 105 (which may have formed therein a plurality of electrical elements such as transistors etc and which may have one or more metal layers). A resistive element is formed from a thin film of resistive material 110. On application of the metal film to the substrate, upper and lower surfaces 115a, 115b are identifiable, the surfaces having enhanced carrier scattering properties. Formed between the two surfaces, a bulk region 120 of the metal film has properties based on the presence of bulk crystallites and grain boundaries within the film. The resistivity and TCR are controlled by the relative amounts of the carrier transport through the bulk crystallites, across the grain boundaries and in relation to the film surface that has a modified construction mechanism.
As shown in FIG. 2 there are observed relationships between TCR and resistivity caused by varying the carrier transport between the surface and the bulk and with different deposition conditions. Deposition conditions such as the type of sputtering gas used, chamber pressure and temperature all will affect in determining the overall characteristics of the formed film. If methods other than sputtering are employed—such as evaporation, chemical vapour deposition etc.—then these too may affect the overall properties of the formed film.
It will therefore be appreciated that there a number of problems associated with forming metal films within semiconductor processes specifically associated with the affect that the processing conditions and environment have on the final properties of the film. A solution would be to apply the metal film as a final layer within the process, thereby minimising its exposure to high temperatures etc. This however is not always a feasible solution as many applications require the metal films to be applied as an intermediary layer. There is therefore a need to provide for the fabrication of metal films resistors in a manner that does not result in a degradation of their TCR or resistive characteristics.